In optical systems such as a camera, etc., there is generally employed a design for “achromatism”, in which glasses having different Abbe's numbers are combined for removing the chromatic aberration of lenses. When glasses that are greatly different in Abbe's number are combined, a great effect is produced on the above removal. In particular, secondary achromatization requires an anomalous partial dispersion glass that has a partial dispersion ratio different from that of a normal optical glass. As an optical glass having a large Abbe's number and having the capability of anomalous partial dispersion, fluorophosphate glasses having an Abbe's number of 80 or more have been put to practical use. Since, however, these fluorophosphate glasses have a refractive index of 1.5 or less, they are not suitable for a lens having large refractive power.
Meanwhile, as an anomalous partial dispersion glass having a refractive index of over 1.5, for example, there is disclosed a fluorophosphate glass having a refractive index of 1.54 to 1.60, an Abbe's number of 68 to 75 and a partial dispersion ratio of at least 0.537 (for example, see JP H04-043854,B). However, this fluorophosphate glass is poor in mechanical properties and thermal properties and in particular has a problem that its abrasion degree is large and that its processability is poor. The processing cost thereof is inevitably high, and it has been difficult to provide high-performance lenses at a low cost.
As a light-weight anomalous partial dispersion glass, further, there is proposed an optical glass having a refractive index of 1.54 to 1.60, an Abbe's number of 70 to 80 and a specific gravity of less than 4.1 (for example, see JP 2003-160356,A). This optical glass is light in weight and excellent in optical properties. Similar to the glass disclosed in JP H04-043854,B, however, it cannot be said that the glass is fully satisfactory in any one of mechanical properties and thermal properties.
Meanwhile, a glass having high refractivity and low dispersion and anomalous partial dispersion capability is liable to have a high liquidus temperature. It is therefore required to produce a low-viscosity glass in order to avoid devitrification. When optical elements are produced from such a glass, however, the problem is that since striae are liable to occur on the surfaces of the optical elements obtained, the yield thereof is liable to be low.